The need for rapid, safe and non-intrusive inspection systems has been increasing greatly. Typical of this need is the necessity of inspecting parcels and packages at transport terminals, post offices and freight depots.
The cost of international travel has decreased significantly recent years and with this decrease there has been a dramatic increase in the volume of international passengers. Concomitant with this increase in the number of travelers has been an increase in the smuggling of contraband via commercial carriers. By contraband it is meant any substance whose trade or transport is restricted by law. As used specifically herein contraband shall refer primarily to hazardous materials such as explosives and narcotics, and shall include military explosives such as mines and ammunition disposed in vehicles, buildings, packages or atop or beneath the surface of the earth.
Because of an increasing demand for narcotics and because of increasing world tension there has been a significant escalation in the transport of such contraband across international borders. It is desirable to interdict such commerce, but because of the greatly increased volume of international traffic, such interdiction becomes very difficult.
While it is theoretically possible to inspect every parcel or item of baggage passing across a border, through an air, rail or sea terminal or through a post office, such inspection would be very costly in terms of wasted time and impeded commerce; furthermore, smugglers frequently resort to the use of packages having hidden compartments, false bottoms and the like which may be overlooked in all but the most scrupulous inspection.
Also, there is a need for explosive detection techniques which may be adapted to quickly scan the surface or subsurface of the earth for mines or other buried explosives. It is preferred that such techniques be reliable, rapid and capable of conducting such inspection from a distance, as for example in a fly-over inspection.
Accordingly, there is a need for a rapid method for detecting the presence of contraband and it is preferred that the technique be noninvasive or nonvisual, that is to say be capable of inspecting the contents of a closed container, without necessitating opening of the container. In many instances a container will not be "closed" in the sense of being sealed, but may be partially opened, porous or permeable; however, as used herein a closed container shall include all such containers not readily subject to visual inspection and shall include parcels, packages, and envelopes as well as structural components of buildings and vehicles. In some instances mines or other military explosives are disposed beneath the soil or waters of the earth's surface, whereas in other instances such devices are "laid" upon the surface of the earth and rely upon camouflage or natural cover to hide them. Therefore, for purposes of this disclosure such devices shall also be deemed to be enclosed.
Such a noninvasive method would save time as well as be highly accurate insofar as false bottoms and the like would not present a source of confusion. It is required that any such inspection technique not harm the articles being inspected or present any lingering physical hazards to the owners of the articles.
Magnetic techniques are employed for the detection of metallic articles such as weapons however, nonmetallic items such as explosives or narcotics are not detected by such techniques. X-ray inspection is frequently employed to view the contents of closed packages, but cannot identify the composition of the observed materials. Many explosives and narcotics have unremarkable X-ray absorption characteristics and hence are indistinguishable from more normal items of commerce. Therefore it may be seen that heretofore employed magnetic or X-ray techniques cannot detect many items of contraband.
The use of neutrons for analysis of the contents of closed packages has previously been investigated. Thermal neutron activation analysis (TNAA) is an analytical technique which has been known and utilized for some time to perform quantitative and qualitative analyses. In this technique relatively low energy neutrons are employed to bombard a sample under investigation. The nuclei of component atoms thereof capture these neutrons and become radioactive. These newly formed radioactive isotopes then undergo atomic decay and emit energetic particles and/or photons in the process. By identifying the emitted radiation, the composition of the sample may be determined. While TNAA techniques are capable of identifying various chemical elements they are not well suited for the high volume inspection of closed containers, as for example at airport terminals, border stations, post offices and the like. TNAA of necessity renders a sample being inspected radioactive and this radioactivity may persist for a significant period of time after completion of analysis thereby presenting a potential health hazard. In many instances, the exact composition of the sample under investigation is not known and consequently the duration of the induced radiation cannot be told beforehand. Thus many inspected items will have the potential of remaining radioactive for fairly long periods of time. Furthermore, TNAA techniques are not particularly efficient for detecting nitrogen or carbon, major components of narcotics and explosives, because the capture cross section for these elements is quite small as compared to that of metals and other heavy elements. Consequently if a usable signal is to be produced, a relatively high neutron flux must be employed, and this high flux can induce significant residual radioactivity in objects being inspected. Additionally, TNAA is very insensitive to oxygen, another element of interest in narcotics and explosives.
Neutron absorption analysis is another technique proposed for the noninvasive inspection of closed containers. In such a process, the absorption of high energy neutrons as they pass through an object is measured. Certain elements are very strongly absorbing of neutrons whereas others are not and this absorption may be utilized to characterize a sample. For example, neutron absorption techniques may be utilized to look for nitrogen or other elements typically associated with narcotics or explosives. The main problem with neutron absorption analysis is that there are a number of elements having very strong absorption signatures which interfere with the detection of the element of interest. For example, boron, as Well as Various rare earth elements have significant neutron absorptions which can mask or otherwise interfere with the absorption of neutrons by nitrogen. Additionally, the absorption of neutrons can create the aforementioned problems of lingering radioactivity.
U.S. Pat. No. 3,997,787 discloses a dual stage analysis system for the detection of explosives in closed packages. The system relies upon the use of thermal neutron activation to detect the presence of oxygen in the contents of the container and neutron absorption to detect the presence of nitrogen therein. The presence of significant quantities of both elements is taken as an indication that explosives may be present in the container. The method of U.S. Pat. No. 3,997,787 suffers from the aforementioned shortcomings of both neutron absorption and neutron activation techniques.
It has now been found that neutron scatter techniques may be adapted for the qualitative and quantitative analysis of the contents of closed packages. Such techniques employ elastically scattered neutrons and as will be explained in greater detail, do not induce significant residual radiation in items and provide for rapid and accurate analysis while minimizing interference from other elements which may be present.
In accord with one embodiment of the present invention it has been found that the resonant elastic scattering of neutrons may be employed with advantage in the detection of contraband in closed containers. Resonant elastic scattering "NRES" is a process whereby neutrons impinge upon and are scattered with minimum energy loss from the nuclei of target atoms. The scattering is typically isotropic insofar as the neutrons are uniformly scattered in all directions from the target nucleus. In those instances where resonant scattering of neutrons is backwards in the general direction of the source, the technique is referred to as back scattering. Since the scattering is elastic no residual radioactivity is created in the target atom. Neutron resonant elastic scattering also has a further advantage in relation to neutron absorption or activation techniques and that is due to the fact that the elastic scattering cross section for neutrons is much larger than the absorption cross section and this difference is greatest for the light elements where scattering cross sections are typically 100 to 1000 times greater than absorption cross sections. Such large cross sections make possible the use of relatively low fluxes of neutrons for resonant elastic scattering analyses.
Neutron resonant elastic scattering techniques are also highly specific for particular elements. That is to say each element has a unique elastic scattering spectrum characterized by the presence of resonance peaks therein, said peaks representing particular neutron energies at which the elastic scattering cross section of a given element is large. Resonance spectra may be readily measured by varying the energy of a monochromatic neutron beam and measuring the intensity of elastically scattered neutrons as a function of beam energy. It should be noted that by the term "monochromatic" is meant a beam having a relatively narrow distribution of energies, analogous to a beam of light of a single wavelength; such a beam may also be referred to as "monoenergetic." As will be explained in greater detail hereinbelow, resonant elastic scattering techniques form the basis for a highly specific and accurate analytical system adapted for the non-invasive interrogation of objects, such as objects which are buried or in closed containers.
Each element has a particular neutron elastic scatter spectrum characterized by a number of particular resonance peaks which can be used to establish the presence of and/or quantify the amount of that particular element present. Likewise, a particular chemical compound will have a unique neutron scatter spectrum reflecting the relative percent of the various component atoms thereof. Contraband items it will thus be appreciated will each present a unique neutron resonant elastic scatter spectrum.
Even more significantly various classes of contraband or other hazardous items will be characterized by certain common features in their resonant scatter spectra, reflecting certain ranges or proportions of various component atoms. For example, explosives broadly fall into two common classes. The first class is referred to as "oxidizing explosives" and its members derive their power from the very rapid oxidation of carbon and hydrogen. Nitrates are the most effective fast acting sources of oxygen for such reactions. Oxidizing explosives are characterized by an oxygen-nitrogen ratio which may range from 1 to 4. Some oxidizing explosives are listed in Table 1 below.
TABLE 1 ______________________________________ Atomic Ratios H:N O:N C:N ______________________________________ Ammonium Nitrate NH.sub.4 NO.sub.3 2.0 1.5 0 Nitroglycerin C.sub.3 H.sub.5 N.sub.3 O.sub.9 1.67 3 1 Nitromethane CH.sub.3 NO.sub.2 3 2 1 Tetranitromethane C(NO.sub.2).sub.4 0 2 0.25 RDX C.sub.3 H.sub.6 N.sub.6 O.sub.6 1 1 0.5 Tetryl C.sub.6 H.sub.5 N.sub.5 O.sub.8 1 1.6 1.2 ______________________________________
A second class of commonly employed explosives derives its power from the high energy disassociation of metastable, oxygen-free nitrogen compounds such as azides. In such explosives, the oxygen-nitrogen ratio is generally 0, the hydrogen-nitrogen ratio is 1 or less and the carbon-nitrogen ratio is lower. Some such explosives are listed in Table 2.
TABLE 2 ______________________________________ Atomic Ratios Dissociating Explosives H:N C:N ______________________________________ Hydrazine Azide N.sub.2 H.sub.4 HN.sub.3 1 0 Guanyl Azide CH.sub.6 N.sub.4 1.5 0.25 Tetrazene CH.sub.7 N.sub.9 O 0.78 .11 ______________________________________
Reference to the ratios listed in the tables above indicates that specific compositional ranges may be associated with specific types of contraband explosives. By reference to a plurality of such compositional ranges false readings which could stem from looking at the ratio of a single pair of elements would be eliminated. For example, acrylonitrile, the basic component of commercial plastics such as Orlon has a carbon-nitrogen of three and hydrogen-nitrogen ratio of three. Based upon a simple analysis for the presence of nitrogen Orlon might be mistaken for an explosive compound. However, the oxygen-nitrogen ratio of acrylonitrile is 0 therefore it can be eliminated as being an oxidizing type explosive. Furthermore, the hydrogen-nitrogen ratio is three, whereas typical disassociating type explosives have a lower hydrogen-nitrogen ratio therefore acrylonitrile can be disqualified as being a dissociating type explosive. Melamine, another common plastic has an empirical formula of C.sub.3 H.sub.6 N.sub.6. Consequently, H:N ratio is 1 and its C:N ratio is 0.5. This might allow it to be confused with RDX; however, the O:N ratio is 0, therefore melamine can be readily distinguished from such explosives.
Neutron resonant scatter analysis enables one to rapidly and reliably obtain a plurality of ratios of elements in a sample and, since what is being measured are ratios and not absolute quantities, the process is effectively "self-standardizing."
Similar ratios may be established for narcotic contraband. Referring now to Table 3, there are shown elemental ratios for particular narcotics.
TABLE 3 ______________________________________ Atomic Ratios Narcotics H:N C:N O:N ______________________________________ Morphine 19 17 3 Cocaine 21 17 4 Heroin 23 21 5 Methadone 27 21 1 Codeine 23 18 3 ______________________________________
It will be seen that there are particular atomic ratios associated with narcotic materials. Referring now to Table 4, there are shown atomic ratios for various articles of commerce which may be expected to be found in luggage, parcels or the like.
TABLE 4 ______________________________________ Atomic Ratios Articles of Commerce H:N C:N O:N ______________________________________ Wool 4.8 3.3 1.1 Silk 4.5 3.0 1.2 Leather 4.8 3.1 1.3 Acrylonitrile 3+ 3+ 0 ______________________________________
It is apparent then that there are distinct groups of atomic ratios associated with explosives, narcotics and innocuous materials and these ratios may be utilized as a basis for the determination of the presence of contraband in a container without the need for the visual inspection thereof.
In addition to the use of a ratio-type analysis, neutron resonant scatter analysis may also be employed to simply determine the presence or absence of particular elements which may be expected to occur in contraband materials. For example, nitrogen is present in virtually all explosives, while mercury, lead or other heavy metals are frequently found in explosive primers. Similarly, sulfur, phosphorous and potassium are frequently found in black powder explosives and elements such as boron and beryllium are present in nuclear devices; thus the presence of such atomic species may be indicative of the presence of explosive contraband.
In an embodiment of this type, a single resonance peak or group of peaks characteristic of a given element is scanned for. Magnitude of the peak will give some information regarding quantities of the species of interest. This single element embodiment is well suited for simple, rapid scanning, and may be used to "pre-screen" items prior to implementing a full ratio type scan.
In accord with another embodiment of the present invention, it has been found that neutron elastic backscatter techniques, "NEBS" which do not necessarily rely upon the use of resonant scattering may also be employed with significant advantage in the inspection of items within closed packages. Neutron elastic backscatter techniques referred to herein as "NEBS" techniques involve the impingement of a monoenergetic beam of neutrons upon an object. The various atoms within the object will scatter the beam, generally in all directions. The scattered neutrons undergo a change in energy and this change will be dependent upon the energy of the incident neutrons, the scatter angle, as well as the atomic mass of the target atoms. It has been found that significant advantage may be enjoyed by analysis of the neutrons which are scattered primarily in the direction from whence they came, such neutrons being referred to as backscattered. The energy of backscattered neutrons is generally given by the formula: ##EQU1## wherein E.sub.Back refers to the energy of the backscattered neutrons, E.sub.In refers to the energy of the incident neutrons and A is the atomic mass of the scattering atom. It will thus be appreciated that for a neutron beam of a given energy there will be a particular energy of backscattered neutron corresponding to the atomic mass of the scattering atom.
Problems have sometimes been encountered in the use of the resonant neutron scatter techniques previously described insofar as resonance peaks of atoms of interest frequently overlap making measurement difficult or impossible. NEBS techniques need not rely upon the measurement of resonance energies; but rather, such techniques analyze the energy/intensity spectrum of neutrons backscattered from an object to determine its composition.
If the energy of the incident beam in a NEBS analysis is varied it will be appreciated that the energy/intensity spectrum of the backscattered neutrons will change. By comparing two spectra made at different energies, both absolute amounts and ratios of particular elements may be determined with minimal interference from overlapping peaks, as will be explained in greater detail hereinbelow.
While the NEBS technique of the present invention need not rely upon resonance measurements it may obviously make use of incident neutron beams having an energy selected to create a resonant backscatter condition for a particular element of interest. The technique may be employed in conjunction with two incident beam-energies, one being of a resonance producing energy and the other being off resonance. Comparison of the backscatter spectra for the two beams will show significant change in the intensity of backscatter neutrons attributable to the element having the resonance energy and by comparison of such spectra, contribution from the resonance producing element may be quantified.
It has been found that scattered neutrons frequently have a significant anisotropic component to their distribution and it has further been found that this component is often at least partially directed, in the backscatter direction. In such instances, the use of backscatter techniques confers additional advantage of enhanced signal intensity. The NEBS technique has also been found advantageous for detecting materials of medium molecular weight such as silicon, sodium, iron, calcium, chromium and nickel.
NEBS techniques may be employed both to determine the ratios of various atomic species in an object or to determine the presence of a single element and in this respect may be employed in a manner quite analogous to, and utilizing the ratios established for, the previously described resonant scattering techniques.
The neutron scatter analysis techniques of the present invention confer significant advantage in the detection of contraband material insofar as such materials may be reliably detected in the presence of relatively large amounts of innocuous substances. Furthermore, the detection can be done without need for opening containers or otherwise unduly delaying commerce. The processes may be readily adapted for scanning structural components of buildings and vehicles as well as for scanning the earth at or just below the surface for buried or submerged objects of interest. In contrast to thermal neutron activation analysis techniques, the techniques of the instant invention do not induce high levels of radioactivity in articles being inspected, and accordingly delay time for "cool down" and hazards to personnel are eliminated. Techniques of the present invention are readily adaptable to full automation and computer control and accordingly can provide for high volume/low cost nonvisual identification of the composition of a variety of items. These and other advantages of the instant invention will be apparent from the drawings, description and claims which follow.